Make copium with dict memo semantically equivalent to stdlib

[Bobronium]

Closes #32

[codspeed-hq]

Edit from @Bobronium: diff is memory allocation noise

## Merging this PR will degrade performance by 13.98%

⚡ 1 improved benchmark
❌ 1 (👁 1) regressed benchmark
✅ 356 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	test_combined_cases[copium-all_reflexive]	33.3 µs	22.2 µs	+49.53%
👁	test_individual_cases[copium-cpython:91610:repeating_atomic]	22.9 µs	26.6 µs	-13.98%

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_{Comparing equivalent-reduce-handling (207cec7) with main (68cd2fe)}